The invention relates to a motor vehicle transmission with a toroidal variable-speed drive unit.
U.S. Pat. No. 6,251,039 B1 discloses such a motor vehicle transmission with a toroidal variable-speed drive unit. This motor vehicle transmission is power-split and has a coaxial construction. The toroidal variable-speed drive unit has two toroidal chambers, the central driven disc of which is connected fixedly in terms of rotation to the power take-off shaft designed as a hollow shaft. By contrast, the input shaft arranged concentrically within the hollow shaft is connected fixedly in terms of rotation to two outer driving discs.
DE 199 47 851 A1 also discloses a toroidal variable-speed drive unit.
An object of the invention is to provide an efficiency-optimized and long-life motor vehicle transmission with a toroidal variable-speed drive unit.
The object is advantageously achieved by the invention described and claimed hereinafter.
One advantage of the invention is that the hollow shaft rotatably mounted with respect to the transmission case is supported axially with respect to the latter. A defined axial position of the hollow shaft is thereby provided at least in an axial direction. Outer axial forces acting on the hollow shaft are supported in the transmission case. Particularly axial forces resulting from the mass inertia of the hollow shaft during the acceleration and braking of the motor vehicle have no influence on the toroidal variable-speed drive unit, but, instead, are supported on the transmission case. Since such mass inertias during acceleration and braking occur particularly in the case of longitudinally installed drive trains, the motor vehicle transmission according to the invention is employed particularly advantageously in such a longitudinally installed drive train. Longitudinally installed drive trains have, in addition, the advantage that particularly high torques can be transmitted by means of these. Particularly in the case of longitudinally installed drive trains in which at least part of the drive torque is transmitted to the rear axle, the front axle can advantageously be provided with high front-wheel steering angles as a result of the low or non-existent drive torques.
Since the outer axial forces no longer have to be taken into account in the regulation of the pressure force between the driving/driven discs and the rollers, the basic pressure and the safety pressure against spinning can be maintained at a low level, this having a positive effect on the efficiency of the toroidal variable-speed drive unit and on the stability of the transmission. Furthermore, a traction fluid on a rolling surface between the driving/driven discs and the rollers is relieved. Also, the regulating operations can advantageously be more accurate.
In addition to the mass inertia, a reason for outer axial forces on the hollow shaft may also be a helical toothing, referred to in Patent Claim 2, on a transmission member of an intermediate transmission, the said transmission member being connected fixedly in terms of movement to the hollow shaft. This helical toothing is advantageously smooth-running, as compared with spur toothings. Such a transmission member may, for example, be a central wheel or a sun wheel.
In certain advantageous embodiments of the invention, the central driven disc or one driving disc is axially displaceable with respect to the hollow shaft. Thus, the two driving discs and the driven disc can be braced relative to the rollers without a high outlay in structural terms, and without the abovementioned advantages of an axially fixed hollow shaft having to be dispensed with. At the same time, the variation in the bracing is particularly advantageous, in order to adapt the pressure between the driving/driven discs and the rollers to the respective torque to be transmitted by the toroidal variable-speed drive unit. This is accompanied by advantages as regards the efficiency and the stability of the toroidal variable-speed drive unit.
In a particularly advantageous embodiment of the invention, a helically toothed transmission member connected fixedly in terms of movement to the hollow shaft can be used even when the helical toothing exerts forces in both axial directions. Such a situation arises, for example, when both forward travel and reverse travel are implemented in one driving range by means of the toroidal variable-speed drive unit, the geared-neutral point lying between the forward travel range and reverse travel range. A reversal in the direction of force likewise takes place in the case of traction/overrun reversal.
By virtue of the axial support of the hollow shaft in both directions mass inertias                both during acceleration        and during braking or,        in the case of transversely installed drive trains, also in right and left cornering travel,have hardly any influence on the hollow shaft and consequently on the toroidal variable-speed drive unit.        
In another particularly advantageous embodiment of the invention, the flexion of the hollow shaft, even under high loads, is kept low. Thus, advantageously, a further radial bearing between the hollow shaft and the radially inner input shaft may be dispensed with in the axial region of the driving/driven disc. In addition to the cost benefits achieved thereby, the radial distance between the hollow shaft and the input shaft can also consequently be kept short, this being accompanied by construction-space benefits.
The rolling bearing in an advantageous embodiment may be designed, for example, as                a single-row grooved ball bearing or        a double-row grooved ball bearing or        a double-row shoulder ball bearing or        a single-row angular ball bearing or        a double-row angular ball bearing or        a double-row conical roller bearing.        
In a further embodiment of the invention, an annular space which is formed between coaxial shafts has a relatively large flow cross section, so that a correspondingly large volume flow can supply the abovementioned bearing with lubricant. Since a large flow cross section also entails low flow losses, the efficiency of the transmission as a whole is particularly high. The need is in this case avoided, in a particularly cost-effective way, for complicated machining operations, in particular cutting machining operations, for producing a separate lubricant duct in one of the shafts.
In a further particularly advantageous embodiment of the abovementioned annular space in conjunction with the above-mentioned radial support. As a result of the radial support of the hollow shaft with respect to the transmission case, a throttle between the hollow shaft and the input shaft can maintain an exactly defined gap. This exactly defined gap then supplies at least one further bearing of the motor vehicle transmission with an exactly defined part-stream of lubricant. Instead of the gap, a sealing ring may also be provided, which allows a defined leakage.
In general, one advantage of power-split motor vehicle transmissions with a toroidal variable-speed drive unit is that, as a result of the use of a power path with a constant step-up, the toroidal variable-speed drive unit is relieved within wide operating ranges. This relief is advantageous particularly in the case of high-torque engines, in which the power take-off torque of the engine is markedly above the maximum permissible input torque of the toroidal variable-speed drive unit and therefore a reduction in the torque of the variable-speed drive unit solely by the preselection of a step-up stage into high speed would not be sufficient. The said high-torque engines are conventionally installed longitudinally in drive trains.
Moreover, along with the corresponding design of the motor vehicle transmission, the relief of the toroidal variable-speed drive unit gives rise advantageously to an improvement in the overall efficiency of the motor vehicle transmission in the corresponding driving range, since the power in the power path having a constant step-up can be transmitted with higher efficiency than in that having a continuously variable step-up.
A further advantage of the relief of the toroidal variable-speed drive unit is that the pressure forces at the driving/driven discs can thereby be lowered, thus leading to a lowering of the frictional losses. As a result of the reduction in the frictional losses, less heat also has to be discharged.
Furthermore, by the toroidal variable-speed drive unit being relieved, its useful life can be increased in an advantageous way.
One advantage of apportioning the transmission step-up to at least two driving ranges is that the spread of the motor vehicle transmission is increased.
Transmission spreads which are greater than the spread of the toroidal variable-speed drive unit thus become possible.
Both driving ranges can advantageously be implemented in the power-split mode, in order to increase the efficiency.
By means of a geared-neutral function, there is advantageously no need for a starting element, such as, for example, a hydrodynamic torque converter. The implementation of a geared-neutral mode makes it possible to have operation in which the driving states forward travel, reverse travel and standstill can be achieved solely by the adjustment of the toroidal variable-speed drive unit. Furthermore, there is no need for a reversing unit, such as, for example, a turning set with associated clutches or brakes, which likewise has an advantageous effect on weight, construction space and costs.
The motor vehicle transmission is used in a particularly advantageous way in a drive train with a front engine and a rear-axle drive. Furthermore, the motor vehicle transmission is used in a particularly advantageous way in an all-wheel drive which emanates from a modified drive train with a front engine and with a rear-axle drive. Such a drive train is shown in DE 101 33 118.5 which has not been published.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.